1. Field of Invention
This invention relates generally to paper-plastic laminate sheeting of high strength, and more particularly to a laminate sheeting in which a paper ply is cold-laminated to a ply formed of an oriented film of synthetic plastic material, the sheeting being foraminated to render it permeable to moisture vapor, but effectively impermeable to liquid.
2. Status of Prior Art
The sheeting traditionally used in making envelopes, grocery bags and other types of dilatable container products which initially are in a flat state is paper. Paper is a semi-synthetic material made by chemically processing cellulose fibers.
Apart from its low cost, an important advantage of paper is that it can be converted into envelopes and other types of dilatable container products by means of high speed equipment that functions to cut and fold the sheeting into the desired configuration, the folds and flaps of the product being bonded together, where necessary, by standard low-cost adhesives. Another advantage of paper in this context is that it can readily be printed and colored, using standard inks for this purpose.
But such paper products suffer from several disadvantages, for they are characterized by low tear and burst strength, and are by no means water resistant; for unless coated, paper is highly absorbent.
Also well known in the art are plastic-coated cellulosic papers, these being used chiefly in children's books, posters, signs and shipping tags, and for other purposes demanding resistance to hard wear and to outdoor exposure. However, such plastic-coated papers lack high tear and burst strength.
To overcome the drawbacks of paper-fabricated dilatable container products, in recent years such products have been made of TYVEK or other polymeric synthetic plastic sheeting. The resultant products not only have a tear and bursting strength far superior to paper, but they are also waterproof.
However, TYVEK and similar synthetic plastic sheeting is difficult to convert into envelopes and other dilatable container products using high-speed equipment of the type mainly suitable for paper. As a consequence, production scrap rates can run as high as thirty percent, thereby raising the cost of manufacturing these products. Moreover, such plastic sheeting has a low chemical affinity for standard adhesives; hence in the case of envelopes, one must then use a special and more costly adhesive on the flaps. And such plastic sheeting also has a low affinity for standard printing inks, and the products, therefore, dictate special printing inks for this purpose.
Disclosed in our above-identified copending application is a paper-plastic laminate sheeting capable of being converted by conventional equipment into envelopes, grocery bags and other dilatable container products that initially are in a flat state and are normally made of paper. The sheeting is composed of a paper facing sheet cold-laminated by means of a water-based adhesive to a reinforcing film of synthetic plastic material, such as polypropylene. The film is oriented to impart exceptional tear and burst strength characteristics to the resultant waterproof product. The product, whose exterior surface is formed by the paper facing sheet, is readily printable. For some applications, a paper facing sheet is cold-laminated to the other side of the film to produce a three-ply laminate sheeting whose exposed surfaces are readily printable and can accept conventional adhesives.
However, a paper-plastic laminate sheeting of the type disclosed in our copending application is not useable in many applications for which TYVEK sheeting is suitable. TYVEK sheeting is breathable, whereas the laminate sheeting is not, for it is impermeable to moisture vapor as well as to liquids. By the term "breathable," as used herein, is meant a sheeting permeable to gases, such as moisture vapor, but not to water and other liquids.
TYVEK is the trademark of the DuPont Company for a non-woven fabric sheeting formed of polyethylene fibers bonded together by heat and pressure to produce a tough, durable sheet structure having minute pores.
Because of its fine porosity, TYVEK sheeting is useable in homes and buildings where the sheeting is attached or adhered to walls, sheathings, door flanges and windows in order to effect energy savings by significantly retarding air leaks and drafts. The fine porosity of TYVEK renders it breathable, allowing potentially damaging moisture vapor to escape. And because the sheeting is water resistant, TYVEK is useable as a barrier under exterior siding.
However, TYVEK sheeting is relatively expensive, for it takes complex machinery to produce fine polyethylene fibers and to convert these into a non-woven fabric sheeting. Moreover, TYVEK sheeting has a low chemical affinity for standard adhesives, and when it becomes necessary to adhere such sheeting to a wall or other surfaces, special and costly adhesives for the purpose are required.
The following patents are of prior art interest in regard to paper-plastic laminates. The British patent 1,569,447 to Steiner, the U.S. Patent to Peer, and the U.S. Pat. Nos. to Williams 4,041,201, 4,351,877, 4,557,971 and 4,554,193. Also of prior art interest in regard to the corona-discharge treatment of plastics are the U.S. Pat. Nos. to Feldmeier et al. 5,180,599 and to Ohno et al. 4,636,427.